Local factors modulate tissue-specific NEFA utilization: assessment in rats using 3H-(R)-2-bromopalmitate.

Insulin-resistant states are associated with accumulation of muscle lipid, suggesting an imbalance between lipid uptake and oxidation. We have employed a new fatty-acid tracer [9,10-3H]-(R)-2-bromopalmitate (3H-R-BrP) to study individual-tissue nonesterified fatty acid (NEFA) uptake in states with diminished or enhanced lipid oxidation. 3H-R-BrP was administered to conscious male Wistar rats (approximately 300 g) during fasting (5, 18, or 36 h), acute blockade of beta-oxidation (etomoxir, 15 micromol/kg), and insulin infusion (0.25 U x kg(-1) x h(-1)). Estimates of NEFA clearance rates (K(f)*) and absolute rates of uptake (R(f)*) were calculated from tissue accumulation of 3H-R-BrP products. In the basal state, NEFA uptake was dependent on the oxidative capacity of tissues: R(f)* in brown adipose tissue (BAT) > heart (HRT) > diaphragm (DPHM) > red quadriceps (RQ) > white quadriceps (WQ) > white adipose tissue (WAT). Fasting increased (P < 0.001) K(f)* in WAT but did not change NEFA clearance in other tissues. However, plasma NEFA levels were raised (P < 0.01), tending to elevate R(f)* in most tissues (P < 0.05: WAT, BAT, WQ, DPHM). Etomoxir reduced (P < 0.01) K(f)* only in oxidative tissues (BAT, RQ, DPHM, HRT). Insulin lowered plasma NEFA levels (P < 0.001) and significantly decreased R(f)* in most tissues (P < 0.05: WAT, RQ, DPHM, HRT). An increased (P < 0.05) clearance was observed in WAT, BAT, and WQ; a decrease (P < 0.01) in K(f)* was observed in HRT. This study is the first to measure tissue-specific NEFA uptake in conscious rats in the postabsorptive, fasted, and insulin-stimulated states. We have demonstrated that tissue NEFA utilization is not exclusively determined by systemic availability, but that the early steps of NEFA uptake or metabolic sequestration can also be rapidly modulated by local processes such as NEFA oxidation.

Effects of individual fatty acids on glucose uptake and glycogen synthesis in soleus muscle in vitro.

Soleus muscle strips from Wistar rats were preincubated with palmitate in vitro before the determination of insulin-mediated glucose metabolism in fatty acid-free medium. Palmitate decreased insulin-stimulated glycogen synthesis to 51% of control in a time- (0-6 h) and concentration-dependent (0-2 mM) manner. Basal and insulin-stimulated glucose transport/phosphorylation also decreased with time, but the decrease occurred after the effect on glycogen synthesis. Preincubation with 1 mM palmitate, oleate, linoleate, or linolenate for 4 h impaired glycogen synthesis stimulated with a submaximal physiological insulin concentration (300 microU/ml) to 50-60% of the control response, and this reduction was associated with impaired insulin-stimulated phosphorylation of protein kinase B (PKB). Preincubation with different fatty acids (all 1 mM for 4 h) had varying effects on insulin-stimulated glucose transport/phosphorylation, which was decreased by oleate and linoleate, whereas palmitate and linolenate had little effect. Across groups, the rates of glucose transport/phosphorylation correlated with the intramuscular long-chain acyl-CoA content. The similar effects of individual fatty acids on glycogen synthesis but different effects on insulin-stimulated glucose transport/phosphorylation provide evidence that lipids may interact with these two pathways via different mechanisms.

Systemic coadministration of chloramphenicol with intravenous but not intracerebroventricular morphine markedly increases morphine antinociception and delays development of antinociceptive tolerance in rats.

Chloramphenicol, an in vitro inhibitor of the glucuronidation of morphine to its putative antianalgesic metabolite, morphine-3-glucuronide (M3G), was coadministered with morphine in adult male Sprague-Dawley rats to determine whether it inhibited the in vivo metabolism of morphine to M3G, thereby enhancing morphine antinociception and/or delaying the development of antinociceptive tolerance. Parenteral chloramphenicol was given acutely (3-h studies) or chronically (48-h studies). Morphine was administered by the i.v. or i.c.v. route. Control rats received chloramphenicol and/or vehicle. Antinociception was quantified using the hotplate latency test. Coadministration of chloramphenicol with i.v. but not i.cv. morphine increased the extent and duration of morphine antinociception by approximately 5.5-fold relative to rats that received i.v. morphine alone. Thus, the mechanism through which chloramphenicol enhances i.v. morphine antinociception in the rat does not directly involve supraspinal opioid receptors. Acutely, parenteral coadministration of chloramphenicol and morphine resulted in an approximately 75% increase in the mean area under the serum morphine concentration-time curve but for chronic dosing there was no significant change in this curve, indicating that factors other than morphine concentrations contribute significantly to antinociception. Antinociceptive tolerance to morphine developed more slowly in rats coadministered chloramphenicol, consistent with our proposal that in vivo inhibition of M3G formation would result in increased antinociception and delayed development of tolerance. However, our data also indicate that chloramphenicol inhibited the biliary secretion of M3G. Whether chloramphenicol altered the passage of M3G and morphine across the blood-brain barrier remains to be investigated.